Where does “calco” come from?

calco (Latin) comes from Latin calx, from Ancient Greek χάλιξ — small stone, pebble; gravel, rubble.

Ancestry of “calco”, step by step

Step	Language	Word	Meaning
1	Latin	calx	limestone; chalk; the finish line
2	Ancient Greek	χάλιξ	small stone, pebble; gravel, rubble